The present invention relates to a frame synchronizer and the switching apparatus that utilizes said frame synchronizer. This invention is appropriate for a wide screen oriented high definition external television signal, such as the next generation Enhanced Definition Television (EDTV), e.g., EDTV-II, when such external television signal is to be frame synchronized with a local television signal.
Since broadcast television cameras in local stations are driven such that color frames thereof are synchronized with each other, no problem occurs with the frame synchronization. In order to switch between the local television signal and a television signal transmitted from outside the local station, a signal switching apparatus must be used in connection with the frame synchronization.
FIG. 5 shows an example of the signal switching apparatus 10. Input terminals 11a-11c receive, for example, local television signals Sa-Sc from three television cameras at the local station. These local television signals are applied to a switcher 14.
An input terminal 13 receives a television signal from outside the local station, i.e., an external signal transmitted through a variety of transmission systems. The television signal from outside the local station (hereinafter referred to as "external television signal") may be a reproduced signal of a VTR that is not synchronized with the television camera.
External television signals are not synchronized with local television signals in general, so a frame synchronizer (FS) 30 is commonly provided between the input terminal 13 and the switcher 14 in order to make the external signal synchronized with the local signal. The output signal from the frame synchronizer is thus applied to the switcher 14 so that the output signal is synchronized with the local television signal. A control central processing unit (CPU) 16 is a control section that applies a read/write signal to the frame synchronizer 30 and applies a switching signal SW to the switcher 14 to control the selection of a target television signal from the four input signals. The control CPU 16 receives an external color frame signal and the like from terminals 13 and 17.
Since a color subcarrier phase returns to the original phase position every four fields in the NTSC system, a color video signal may be delayed so as to match the color frame phases of independent video signals every four fields. This results in a large time difference between the video signal and an audio signal. In a synchronous relay of the broadcasting of a connection between the local station and an outside station, the time difference between the audio and video signals is conspicuous and viewers may feel uncomfortable.
In consideration of the above discussed points, a conventional frame synchronizer 30 delays the video signal for two fields and shifts it by two pixels. Thus, the frame synchronizer can reduce the incompatibility at the synchronous relay and can synchronize the external television signal with the local one. In this instance, a minimum delay value may be applied to the video signal.
The above discussed case is an example of a color video signal being digitized with a clock frequency of 4 fsc (fsc is a color subcarrier frequency). It takes four pixel periods for the color phase to return to its original phase position in the 4 fsc digitizing operation. Thus, if the video signal is delayed by two fields, the color phase may be shifted by one hundred and eighty degrees. When the color phase is shifted by one hundred and eighty degrees and further shifted by two pixel periods (corresponding to one hundred and eighty degrees), only a two-field delay process of the external television signal will cause the color phase of the external signal to match the local color phase.
For the foregoing operation, the frame synchronizer 30 writes the digitized television signal with a write clock WCK that is synchronized with the external television signal and reads the stored signal with a read clock that is synchronized with the local television signal. In this instance, the above discussed readout timing is maintained.
Such a frame synchronization process cannot be applied to all television signals. Especially when the high definition oriented television signal is used as the external television signal, the above described conventional process cannot be applied to such a signal without modification.
The television broadcasting signal currently used is an interlacing signal. The EDTV signal which has 525 scanning lines per field (for the NTSC system) is known as one of the high definition television signals that maintains compatibility with the interlacing signal.
In the EDTV system, a high-vision broadcasting system is being proposed for a wide screen oriented television system (what is called the next generation EDTV system, i.e., EDTV-II).
The screen aspect ratio of the high-vision broadcasting system is 16:9. The EDTV-II system has the same aspect ratio. Since the EDTV-II system is oriented to high definition while maintaining compatibility with the present television system, the television signal of the EDTV-II system can be applied to the screen of the current aspect ratio (4:3) by selecting a signal type. For example, a letter box type signal is appropriate for maintaining compatibility when the upper and lower parts of the screen are blank areas that include no information.
FIG. 6 shows a screen configuration in which the EDTV-II video signal is displayed on a conventional screen 20 having the present aspect ratio (4:3). The main screen area 21 is positioned at the center of the screen. When the video image of the EDTV-II system is displayed on the screen 20 having the current aspect ratio, the image in the main screen area 21 corresponds to the EDTV-II video image.
The screen 20 includes portions above and below the main screen area 21 called the upper blank area 22A and the lower blank area 22B which contain no information. These blank portions are processed as non-information areas (actually a gray that is almost black) so that the compatibility can be maintained with a video system having the current aspect ratio.
A video signal for the upper and lower blank areas 22A and 22B is called a support (reinforcement) signal that is used in the high definition mode. This signal may be a horizontal high-band signal or a vertical high-band signal.
The main screen area 21 consists of one hundred and eighty horizontal lines and each of the upper and lower blank areas 22A and 22B consists of thirty horizontal lines. Since the support signal in the video signal for the main screen area should be assigned to the sixty horizontal lines comprising the upper and lower blank areas, the support signal is compressed one third and is inserted into the upper and lower blank areas 22A and 22B.
In FIG. 6, each of the upper and lower blank areas 22A and 22B is divided by three horizontally. For example, the support signal for the upper ninety horizontal lines in the main screen area 21 is inserted into the upper blank area 22A. As shown in FIG. 7, the support signal (compressed image data) for the first to thirtieth horizontal lines in the main screen area is inserted into the divided area 23A, the support signal for the thirty-first to sixtieth horizontal lines in the main screen area is inserted into the divided area 23B, and the support signal for the sixty-first to ninetieth horizontal lines in the main screen area is inserted into the divided area 23C in order to form the upper blank area 22A. For the lower blank area 22B, the support signal for the lower ninety horizontal lines in the main screen area is processed in a fashion similar to the support signal for the upper blank area.
If reference numbers representative of pixel positions of the main screen signal are used to indicate the pixel positions of the corresponding support signal, the horizontal pixel position in the main screen area 21 does not match that of the support signal that is to be inserted into the divided area in the upper blank area 22A as shown in FIG. 7. This is due to the compression process of the support signal and the scattering insertion process for the divided areas as previously discussed.
Since the pixel positions of the upper blank area and the main screen area do not match, a pixel relationship between the main screen signal and the support signal cannot be maintained when the external television signal is processed in the frame synchronization operation which synchronizes the external color frame with the local color frame, especially when the two-pixel shift process is performed.
FIG. 8 shows a data diagram explaining the above discussed process wherein FIG. 8A represents the pixel position of the support signal. When this support signal is compressed one third, the compressed signal becomes the signal shown in FIG. 8B. When this compressed support signal is decoded (processed by extending it), the original support signal is restored as shown in FIG. 8C. A predetermined signal process is accomplished by, e.g., applying the restored support signal to the main screen signal, thereby reproducing the high definition video.
The two-pixel shift process is necessary for the frame synchronization as previously described. This two-pixel shift process is equivalent to a special readout process wherein a meaningless signal (represented by "x") is the output for two pixels and the actual compressed support signal is the output shown in FIG. 8D.
In such a restoring process for the compressed support signal, the first output pixel is processed as the first compressed support signal shown in FIG. 8B and then the signal is restored as shown in FIG. 8E. However, the restoring process should be undertaken with an actual two-pixel shift. The restoring process should be undertaken as shown in FIG. 8F. FIG. 8E represents an incorrect restoring process.
What is desired is a frame synchronizer and a signal switching apparatus that utilizes the frame synchronizer to solve the abovediscussed problems. With such devices the restoring process can be done correctly even if a high definition television signal, such as EDTV-II signal, is applied as the external television signal.